The invention lies in the field of recovering aluminum from its ores by pyrometallurgical and crystallization techniques.
The present invention is an improvement in the basic process disclosed in the above-referenced and incorporated patent for recovering aluminum from alunite ore, the basic process broadly comprising dehydrating the particulate ore by roasting, removing sulfur and alkali metal compounds from the roasted ore by a reducing roast followed by an optional oxidizing roast and leaching with water as a solvent, resulting in a residue containing aluminum values and a solution containing potassium, sulfate, which may optionally be recovered, digesting the residue with alkaline solution to convert the aluminum values to soluble aluminates, removing silicon from the aluminate solution, and precipitating the aluminum values as aluminum hydroxide from which alumina is recovered.
The invention is an improvement in the above basic process in which a two stage reduction of the dehydrated ore is utilized, sulfur being used as the reducing agent in the first stage and a different reducing agent such as hydrogen being used in the second stage.
As used herein the term "Bayer leach" refers to digestion with at least on alkali metal hydroxide of the residue resulting from the roasting and leaching of the alunite ore as discussed above to remove compounds of sulfur and alkali metals.
In the processing of the calcined alunite ore prior to the Bayer leach, there must be a clean separation of aluminum values and sulfates and the processing, particularly the reduction procedure, must not affect the aluminum content of the ore to reduce the effectiveness of the Bayer procedure. If the sulfates are not completely removed, they will build up to intolerable levels in the Bayer leach circuit as they are soluble in the Bayer leach liquor.
In order to insure a maximum removal of sulfur and alkali metal compounds from the residue with maximum recovery of aluminum it is imperative that the reduction step be as effective as can be obtained. The economical water leaching system demands complete reduction in order to complete the potassium sulfate extraction and produce a sulfur free Bayer feed. A straight hydrogen reduction system produces a completely reduced alunite but it is too expensive for a competitive commercial operation.
Accordingly, it is an object of this invention to provide an improved process for recovering aluminum from alunite ore in which an effective reduction of the ore is obtained with improved economics, resulting in effective separation of sulfate from the dehydrated ore with maximum recovery of aluminum.